Thermally enhanced molded package for semiconductors

FIELD OF THE DISCLOSURE

0001The present disclosure relates generally to the fabrication of semiconductor devices, and relates more particularly to the packaging of semiconductor devices and to the thermal enhancement of such packaging.

BACKGROUND OF THE DISCLOSURE

0002Integrated circuits (ICs) generate heat as an undesirable byproduct during their use. This heat byproduct is a significant design consideration, both in the design of the IC and in the design of products incorporating the IC. One strategy for addressing this issue is to design integrated circuits so that they will generate less heat in the first place. Another strategy for addressing this issue is to control how the IC is driven during use. A further strategy is to cool the IC.

0003Many different tactics have been used to cool ICs. One tactic is to convectively cool the IC by applying moving air over it. This may involve, for example, the use of heat exchangers filled with liquids or gasses to remove heat generated by the IC. Another common tactic to cool ICs has been to attach a heat sink with radiant cooling fins to the IC. In such a device, the cooling fins act to wick away the heat generated by the IC. Frequently, the finned heat sink is combined with a fan, in which case the device radiates off heat or, by acting in conjunction with the fan, convectively transfers heat.

0004In most implementations of the tactics described above, the ICs have been housed in mechanically designed housings called IC packaging. The packaging provides the dual functionality of physically and electrically insulating the IC, while at the same time providing easy electrical contact to the IC. Since the primary purpose of the packaging is to insulate and protect the IC, the packaging has a tendency to inhibit the release of heat energy.

0005Prior art tactics employed to address this issue include the incorporation of a heat spreader inside the packaging, or the inclusion of a heat slug which has a surface exposed on the outer surface of the package. <figref idref="DRAWINGS">FIG. 1</figref> illustrates a cross-section of a prior art surface mounted IC package employing a heat slug. This example is a thermally enhanced plastic ball grid array (TE PBGA) package. The IC package <b>10</b> is a housing for an IC <b>12</b> with an active surface <b>14</b> and a non-active surface <b>16</b>, and which is affixed via an adhesive layer <b>18</b> to a substrate <b>20</b>. The active surface <b>14</b> of the IC <b>12</b> is electrically connected via wire <b>22</b> to mounting surfaces <b>24</b> on the substrate <b>20</b>. These mounting surfaces <b>24</b> contain traces <b>26</b> which are typically eventually electrically connected to solder balls <b>28</b>. Though not shown in <figref idref="DRAWINGS">FIG. 1</figref>, these solder balls <b>28</b> are the means by which the packaged IC <b>10</b> is surface mounted to provide electrical connection to an electronic circuit board (not shown).

0006In the IC package shown, the heat slug <b>30</b> is non-electrically mounted via mounts <b>32</b> on the substrate <b>20</b> over the IC <b>12</b>, and is surrounded by the molding compound <b>34</b>. The slug is typically manufactured out of copper, aluminum, or steel. In the case of the particular TE PBGA depicted, the slug is approximately 300 μm thick and is made of copper. In some prior art IC packages, the heat slug <b>30</b> remains exposed as shown in <figref idref="DRAWINGS">FIG. 1</figref> so that it can transfer heat more easily out of, and away from, the IC and its packaging. Other prior art IC packages employ a heat spreader fully encased in the molding compound. The heat slug configuration provides marginal beneficial effects. However, the benefits of heat slugs <b>30</b> configured in this way have limited efficiency and effectiveness, and are problematic to manufacture.

0007Part of the reason for its limited effectiveness is that the heat slug is too far from the IC <b>12</b> heat source. The packages illustrated in <figref idref="DRAWINGS">FIG. 1</figref> are typically rated to handle two to three watts (2 W-3 W). Additionally, the space between slug <b>30</b> and IC <b>12</b> is typically filled with molding compound <b>34</b>. Because of its low thermal conductivity, the molding compound acts to thermally insulate the IC <b>12</b>. This fact may be appreciated from TABLE 1 below, which compares the thermal conductivity of a typical molding compound such as G760 with the thermal conductivities of some other materials typically present in the device.

0008<tables id="TABLE-US-00001" num="00001"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="3"><colspec colname="offset" colwidth="14pt" align="left" /><colspec colname="1" colwidth="70pt" align="left" /><colspec colname="2" colwidth="133pt" align="center" /><thead><row><entry /><entry namest="offset" nameend="2" rowsep="1">TABLE 1</entry></row><row><entry /><entry namest="offset" nameend="2" align="center" rowsep="1" /></row><row><entry /><entry>Material</entry><entry>Thermal Conductivity (W/mK)</entry></row><row><entry /><entry namest="offset" nameend="2" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry /></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="3"><colspec colname="offset" colwidth="14pt" align="left" /><colspec colname="1" colwidth="70pt" align="left" /><colspec colname="2" colwidth="133pt" align="char" char="." /><tbody valign="top"><row><entry /><entry>Silicon</entry><entry>148.0</entry></row><row><entry /><entry>Copper</entry><entry>386.0</entry></row><row><entry /><entry>Aluminum</entry><entry>222.0</entry></row><row><entry /><entry>37-63 Solder</entry><entry>50.7</entry></row><row><entry /><entry>molding compound</entry><entry>0.7~0.9</entry></row><row><entry /><entry namest="offset" nameend="2" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

0009The lack of proximity between the slug <b>30</b> and the IC, and the thermally insulating properties of the molding compound, limit the efficiency of removing heat from the IC <b>12</b>. While it may be possible to move the slug <b>30</b> closer to the IC, this creates design issues because, in such a position, the slug can interfere with the wire leads <b>22</b> to the IC <b>12</b> and the substrate circuit <b>26</b>, and complicates the formation of the molding compound <b>34</b> around the IC <b>12</b> and the slug <b>30</b>.

0010Another limitation of some prior art heat slugs is that they require the use of mounts <b>32</b> to maintain distance between the slug <b>30</b> and the IC <b>12</b>. The use of mounts <b>32</b> is undesirable in that they require landing areas (not shown) on the substrate which must be accommodated when designing the substrate circuits <b>26</b>.

0011<figref idref="DRAWINGS">FIG. 2</figref> is an illustration of another prior art IC package <b>40</b>. In this package, an interposer layer <b>42</b> rests between the IC <b>12</b> and the heat slug <b>44</b>. Typically, in such a package, a conductive adhesive layer <b>46</b> is disposed between IC <b>12</b> and interposer layer <b>42</b>, and a conductive adhesive layer <b>48</b> is disposed between the interposer layer <b>42</b> and the heat slug <b>44</b>.

0012Although the package in <figref idref="DRAWINGS">FIG. 2</figref> has better thermal properties, it is difficult to manufacture. In particular, if the accumulated vertical dimensions are on the larger side of acceptable tolerances, when the device is clamped in the mold tool, the IC may be crushed or damaged. If the dimensions are adjusted to avoid crushing of the die, the mold compound will tend to extend over the top of the spreader, thus leading to poor thermal performance. Hence, the tolerance accumulated from the thicknesses of the components and adhesive layers will cause the manufacturing process to vary between damage due to crushing and poor thermal performance or molding placement.

0013There is thus a need in the art for removing heat from an IC that is more efficient and/or easier to manufacture, and is therefore more effective. These and other needs are met by the devices and methodologies described herein. Based on a preliminary analysis, it is believed that the improvements to IC packaging described below may produce a doubling of the wattage capacity of the packaging.

BRIEF DESCRIPTION OF THE DRAWINGS

0014The following is a brief description of the drawings and should be considered in conjunction with the preceding and following detailed description:

0015<figref idref="DRAWINGS">FIG. 1</figref> is an illustration of a prior art IC package with a heat slug;

0016<figref idref="DRAWINGS">FIG. 2</figref> is an illustration of a prior art IC package with a non-compressive interposer disposed between the IC and the heat slug;

0017<figref idref="DRAWINGS">FIG. 3</figref> is an illustration of an embodiment of an IC package with a Z-shaped, compressive interposer disposed between the IC and the heat slug;

0018<figref idref="DRAWINGS">FIG. 4</figref> is an illustration of an embodiment of an IC package with a Z-shaped, compressive interposer without a heat slug;

0019<figref idref="DRAWINGS">FIG. 5</figref> is an illustration of an embodiment of an IC package with a Z-shaped, compressive interposer affixed to the active surface of the IC;

0020<figref idref="DRAWINGS">FIG. 6</figref> is an illustration of a flip chip package equipped with a four-leaf, Z-shaped compressive interposer;

0021<figref idref="DRAWINGS">FIG. 7</figref> is an illustration of an embodiment of a Z-shaped compressive interposer;

0022<figref idref="DRAWINGS">FIG. 8</figref> is an illustration of the embodiment of a Z-shaped compressive interposer which is illustrated in <figref idref="DRAWINGS">FIG. 7</figref>, prior to shaping for assembly;

0023<figref idref="DRAWINGS">FIG. 9</figref> is an illustration of a four-leaf Z-shaped compressive interposer in a flip chip package, prior to shaping for assembly;

0024<figref idref="DRAWINGS">FIG. 10</figref> is an illustration of the compressive interposer in <figref idref="DRAWINGS">FIG. 9</figref>, after folding;

0025<figref idref="DRAWINGS">FIG. 11</figref> is an illustration of an embodiment of an accordion-shaped, compressive interposer;

0026<figref idref="DRAWINGS">FIG. 12</figref> is an illustration of the embodiment of the accordion-shaped, compressive interposer which is illustrated in <figref idref="DRAWINGS">FIG. 11</figref>, prior to shaping for assembly;

0027<figref idref="DRAWINGS">FIG. 13</figref> is an illustration of an embodiment of a C-shaped, compressive interposer;

0028<figref idref="DRAWINGS">FIG. 14</figref> is an illustration of the embodiment of the C-shaped, compressive interposer which is illustrated in <figref idref="DRAWINGS">FIG. 13</figref>, prior to shaping for assembly;

0029<figref idref="DRAWINGS">FIG. 15</figref> is an illustration of a four-leaf, compressive interposer before it is folded;

0030<figref idref="DRAWINGS">FIG. 16</figref> is a top view illustration of the four-leaf, compressive interposer of <figref idref="DRAWINGS">FIG. 15</figref>, after it is folded;

0031<figref idref="DRAWINGS">FIG. 17</figref> is a side view illustration of the four-leaf, compressive interposer of <figref idref="DRAWINGS">FIG. 15</figref>, after it is folded;

0032<figref idref="DRAWINGS">FIG. 18</figref> is an illustration of an alternative embodiment of a compressive interposer;

0033<figref idref="DRAWINGS">FIG. 19</figref> is an illustration of the embodiment of the compressive interposer which is illustrated in <figref idref="DRAWINGS">FIG. 18</figref>, prior to shaping for assembly;

0034<figref idref="DRAWINGS">FIG. 20</figref> is an illustration of an alternative embodiment of a compressive interposer;

0035<figref idref="DRAWINGS">FIG. 21</figref> is an illustration of the embodiment of the compressive interposer which is illustrated in <figref idref="DRAWINGS">FIG. 20</figref>, prior to shaping for assembly;

0036<figref idref="DRAWINGS">FIG. 22</figref> is an illustration of a combination of different shaped compressive interposers;

0037<figref idref="DRAWINGS">FIG. 23</figref> is an illustration of a multi-chip IC package; and

0038<figref idref="DRAWINGS">FIG. 24</figref> is an illustration of a compressive standoff for supporting a compressive interposer over the substrate.

DETAILED DESCRIPTION

0039<figref idref="DRAWINGS">FIG. 3</figref> illustrates an IC package <b>50</b> with improved heat transmission characteristics. The IC <b>12</b> in package <b>50</b> is thermally connected to the heat slug <b>36</b> by means of a shaped compressive interposer <b>52</b>. In the embodiment illustrated in <figref idref="DRAWINGS">FIG. 3</figref>, the compressive interposer <b>52</b> has a Z-shaped compressive section described in greater detail below.

0040The core of the IC package <b>50</b> is the IC <b>12</b>. The IC <b>12</b> is also the primary source of heat which must be dissipated. Typically, ICs are silicon based structures of complex manufacture with many fine details, and are relatively fragile. This is one of the reasons they must be enclosed in a package. A typical IC at the time of authorship is 50-360 μm thick. It is usually configured in a package with its active surface <b>14</b> up, and is affixed to a substrate <b>20</b>.

0041The substrate is similar to a circuit board in that it contains circuitry <b>26</b> that routes the connections made by means of connectors <b>22</b> to surface-mount solder ball <b>28</b> locations. The substrate <b>20</b> may be rigid. Currently typical rigid substrates range in thickness from approximately 200 μm to a more standard 360 μm or 560 μm. The substrate could also be a film substrate approximately 50 μm to 100 μm thick.

0042The embodiment illustrated in <figref idref="DRAWINGS">FIG. 3</figref> employs a non-compressive interposer <b>68</b>. A typical non-compressive interposer <b>68</b> might be approximately 50 μm to 200 μm in thickness and typically comprises a blank silicon die to match the coefficient of expansion of the IC <b>12</b>. The non-compressive interposer <b>68</b> is typically bonded to the IC with a thermally conductive adhesive (not shown) which is also electrically insulating so as to avoid shorting the active side <b>14</b> of the IC <b>12</b>.

0043In the embodiment illustrated in <figref idref="DRAWINGS">FIG. 3</figref>, the compressive interposer <b>52</b> is mounted on top of the non-compressive interposer <b>68</b>. In such an embodiment, a silver filled adhesive (not shown) can be used because there is no risk of electrical conductance. The use of a silver filled adhesive is also advantageous in that it has higher thermal conductance properties. In the embodiment illustrated, the compressive interposer comprises a shaped 150 μm copper sheet. In other embodiments, other materials and thicknesses and shapes may be used. The choice of materials and material thicknesses depends on a number of factors. Some of these factors include how much heat needs to be transmitted and at what rate, the flexibility of the material, the mechanics of the clamping and molding process, and the configuration of the package. Generally, the overall requirements are that the material is thermally conductive, and that the structure is deformable during the molding process and provides enough surface area on the top and bottom to collect and transmit the heat generated by the IC <b>12</b> to an external heat sink (not shown).

0044The purpose of the heat slug <b>36</b> is to act as a heat sink and to provide a thermal connectivity path for other heat transfer devices (not shown) when the IC package is used on circuit boards (not shown) for devices requiring an IC. Many suitable materials may be used as the heat slug including, but not limited to, silicon, aluminum, steel, copper and other heat transmissive materials. It is desirable that the coefficient of thermal expansion of the slug <b>36</b> material be closely matched with the coefficient of thermal expansion of its surrounding materials to avoid failure caused by thermal cycling of the package <b>50</b>.

0045In this embodiment, the top section <b>52</b> of the compressive interposer makes thermal contact with the heat slug <b>36</b>. While, in some embodiments, a thermally conductive adhesive or paste may be placed between the compressive interposer <b>52</b> and the heat slug <b>36</b>, it is preferred that this material be soft, or at least flexible. The reason for this preference is to allow for variation in the position of the compressive interposer <b>52</b> relative to the heat slug <b>36</b> during manufacture. The center section <b>54</b> of the compressive interposer <b>52</b> makes thermal contact with a non-compressive interposer <b>68</b>. In this interface a thermally conductive adhesive (not shown) may be used. The non-compressive interposer <b>68</b>, in turn, is thermally mounted on the active surface <b>14</b> of the IC <b>12</b>. Between these layers, a non-electrically conductive, thermally conductive adhesive (not shown) is typically used. This non-compressive interposer layer can be made of any thermally conductive material. However, since it is in close proximity to the active surface of the IC, it is preferred that this material is a non-electrically conductive material such as a blank silicon die. In the embodiment shown in <figref idref="DRAWINGS">FIG. 3</figref>, the non-compressive interposer layer is a blank silicon die designed to provide electrical insulation from the compressive interposer <b>52</b>. Controlled thermal expansion materials, such as molybdenum, invar-copper-invar or copper-invar-copper structures, may be preferred in some embodiments, depending on cost and need.

0046During operation, the heat generated by the IC <b>12</b> is transmitted from the IC <b>12</b> through the non-compressive interposer <b>68</b>, to the center section <b>54</b> of the compressive interposer <b>68</b>, and up to the top section <b>52</b> via a compressive section <b>64</b> of the compressive interposer <b>52</b>. The compressive section <b>64</b> of the compressive interposer <b>52</b> includes bends <b>62</b> and <b>60</b> of the compressive interposer <b>52</b>. The compressive nature of the compressive section <b>64</b> is particularly advantageous during manufacture of the package <b>50</b>. Specifically, the compressive section <b>64</b> allows compressive forgiveness while still maintaining thermal contact between the IC <b>12</b> and heat slug <b>36</b>. A secondary advantage of the compressive section <b>64</b> is that it is believed that this flexibility will allow for greater tolerance to differences in the coefficient of expansion of adjacent materials during thermal cycling.

0047A suitable material for the compressive interposer <b>52</b> has high thermal conductivity and is flexible in the desired shape. Examples of suitable materials include copper, aluminum and steel and flexible ceramics. The choice of material and thicknesses, and compressive section details with this and other configurations described herein, depend on the needs of a particular package. Although <figref idref="DRAWINGS">FIG. 3</figref> illustrates a shaped compressively deformable interposer, other types of compressive interposers would also be suitable in other applications. For example, the compressive interposer could be wire mesh or engineered materials with microstructures, such as thermally conductive foams. The salient characteristics of the materials are that they provide good thermal conductance and are compressively deformable during the IC packaging process.

0048<figref idref="DRAWINGS">FIG. 4</figref> is an illustration of an IC package <b>51</b> similar to the package in <figref idref="DRAWINGS">FIG. 3</figref>, with the exception that it is lacking the slug/spreader <b>36</b> of <figref idref="DRAWINGS">FIG. 3</figref>. In this embodiment, the outer surface <b>58</b> of the compressive interposer <b>52</b> is exposed to provide direct contact with external heat transfer devices (not shown).

0049<figref idref="DRAWINGS">FIG. 5</figref> is an illustration of an IC package <b>53</b> similar to the IC package <b>51</b> of <figref idref="DRAWINGS">FIG. 4</figref>. The difference in the package illustrated in <figref idref="DRAWINGS">FIG. 5</figref> is the absence of the non-compressive interposer <b>68</b> in <figref idref="DRAWINGS">FIG. 4</figref>. In this embodiment, the center section <b>54</b> of the compressive interposer <b>52</b> is affixed directly to the active surface <b>14</b> of the IC <b>12</b> via an electrically non-conductive, thermally conductive adhesive (not shown). In one embodiment, the adhesive takes the form of a thin tape. Thermally conductive adhesives are widely available in many forms, including tape forms and softer paste forms. Both electrically conductive and electrically non-conductive adhesives that suit the purposes described herein are widely available.

0050<figref idref="DRAWINGS">FIG. 6</figref> illustrates an IC package <b>70</b> with a flipped chip. In this package, the active surface <b>14</b> of the IC <b>12</b> is down. The IC <b>12</b> is electrically attached to the substrate <b>20</b> via solder balls <b>29</b>. It should be noted that the IC package <b>70</b> shown in <figref idref="DRAWINGS">FIG. 6</figref> employs a four-leaf, compressive interposer <b>71</b>. Three (3) compressive sides <b>84</b> can be seen in <figref idref="DRAWINGS">FIG. 6</figref>. This embodiment of a compressive interposer is described in greater detail below.

0051<figref idref="DRAWINGS">FIGS. 7-8</figref> illustrate the compressive interposer <b>52</b> of <figref idref="DRAWINGS">FIGS. 3-5</figref> after they are bent into shape. When the compressive interposer is incorporated into a device, the top section <b>58</b> will be in thermal contact with the heat slug <b>36</b> in <figref idref="DRAWINGS">FIG. 3</figref> and <figref idref="DRAWINGS">FIG. 5</figref>, and will be exposed to the outside of the package in <figref idref="DRAWINGS">FIG. 4</figref>. The center section <b>54</b> will be in thermal contact with the non-compressive interposer <b>68</b> in <figref idref="DRAWINGS">FIGS. 3-4</figref> and with the IC in <figref idref="DRAWINGS">FIG. 5</figref>. Fold lines <b>60</b> and <b>62</b> create the compressive section <b>64</b> which allows for compressive give during the manufacturing process and during thermal cycling.

0052<figref idref="DRAWINGS">FIG. 8</figref> also illustrates two additional features that are only shown in this embodiment but may be applied to any other embodiment as well. These include flow holes <b>66</b> to allow the flow of molding compound during manufacture, and locking features <b>69</b> to help hold the compressive interposer <b>52</b> in place during its life.

0053<figref idref="DRAWINGS">FIGS. 9-10</figref> illustrate another embodiment of a compressive interposer <b>71</b>. This compressive interposer is similar to the two-leaf interposer <b>52</b> in <figref idref="DRAWINGS">FIGS. 7-8</figref>. Though the illustrated interposer is only shown in <figref idref="DRAWINGS">FIG. 6</figref>, it would apply to the other package configurations as well. The center section <b>74</b> is affixed to the IC <b>12</b> or non-compressive interposer (not shown), depending on the package design. The compressive sections <b>84</b> fold up so that the second fold line <b>80</b> moves closer to the center of the center section <b>74</b> of the compressive interposer <b>71</b> along fold lines <b>82</b>, after which the top sections <b>72</b> fold back down along fold lines <b>80</b> to a horizontal position in a plane parallel to the plane of the center section <b>74</b>. These top sections are then either affixed to a heat slug (not shown) exposed to the outside of the package (not shown), or are exposed to the outside of the package (not shown) ready to receive external heat transfer systems (not shown).

0054<figref idref="DRAWINGS">FIG. 11</figref> illustrates an alternative embodiment of the compressive interposer <b>102</b>. In this embodiment, the compressive section of the compressive interposer <b>102</b> is formed by multiple bends <b>104</b>, <b>106</b>, <b>108</b> and by straight subsections <b>112</b> and <b>114</b>, thereby forming an accordion type configuration. It is believed that this configuration may allow the bottom section <b>110</b> of the interposer <b>102</b> to remain flat on the IC or non-compressive interposer <b>116</b>, with the choice depending on the configuration of the rest of the package and on whether a flip chip configuration is desired. For example, in a flip chip configuration, a non-compressive interposer may not be used. On the other hand, if the active surface of the chip is facing upward, it might be desirable for the bottom surface <b>120</b> of a non-compressive interposer <b>116</b> to be facing the active surface of the IC rather than the compressive interposer, particularly if the compressive interposer is made of an electrically conductive material. <figref idref="DRAWINGS">FIG. 11</figref> further illustrates the use of two interposers <b>102</b>.

0055<figref idref="DRAWINGS">FIG. 12</figref> illustrates one of the compressive interposers <b>102</b> of <figref idref="DRAWINGS">FIG. 11</figref> prior to being folded for assembly. Top section <b>102</b> will be exposed or will be positioned so that it is in thermal contact with a heat slug (not shown). Bottom section <b>110</b> will be contacting the non-compressive interposer or IC <b>116</b>. The compressive section will be formed using straight sections <b>112</b> and <b>114</b> and bend lines for bends <b>104</b>, <b>106</b> and <b>108</b>.

0056<figref idref="DRAWINGS">FIG. 13</figref> illustrates an alternative embodiment of a compressive interposer <b>132</b> for the present disclosure. In this embodiment, the compressive section <b>134</b> is C-shaped. <figref idref="DRAWINGS">FIG. 14</figref> illustrates the top section <b>132</b> which will be exposed or in thermal contact with a heat slug, and the bottom section <b>136</b> which will be in thermal contact with the IC or non-compressive interposer <b>116</b> with a bottom IC facing surface <b>120</b>. <figref idref="DRAWINGS">FIG. 14</figref> also illustrates the lines that represent the beginning and end of the C-shaped curvature of the compressive section <b>134</b>.

0057<figref idref="DRAWINGS">FIGS. 15-17</figref> illustrate an embodiment of a compressive interposer <b>250</b> which combines elements of an accordion-shaped, compressive section with a Z-shaped, compressive section. <figref idref="DRAWINGS">FIG. 15</figref> illustrates the compressive interposer <b>250</b> before it is folded. <figref idref="DRAWINGS">FIG. 16</figref> illustrates the compressive interposer <b>250</b> after it is folded. <figref idref="DRAWINGS">FIG. 17</figref> illustrates the manner in which the compressive section is folded into an accordion-like shape. One advantage of this design over the design illustrated in <figref idref="DRAWINGS">FIGS. 9-10</figref> is that there is less of a bottle neck in the compressive sections. Another advantage is that it creates more surface area in the top sections <b>252</b> to affix to a heat slug (not shown) or exposed outside the package (not shown) to affix to an external heat transfer device (not shown).

0058To fold the compressive interposer illustrated in <figref idref="DRAWINGS">FIG. 15</figref>, the top sections <b>252</b> are first folded up along bend <b>260</b>. Then, the sides of compressive section <b>264</b> are folded back so that their centers fold along fold lines <b>266</b>, while the compressive interposer is simultaneously folded along fold lines <b>262</b> to get an accordion-shape as illustrated in <figref idref="DRAWINGS">FIG. 17</figref>. The triangular shaped section <b>265</b> becomes the center of the Z-shape section of the design illustrated in <figref idref="DRAWINGS">FIG. 15</figref>.

0059<figref idref="DRAWINGS">FIGS. 18-19</figref> illustrate an alternative embodiment of the C-shaped spring interposers illustrated in <figref idref="DRAWINGS">FIGS. 12-13</figref>. In this embodiment, the interposers are formed from a series of interleaved springs with C-shaped compressive sections <b>144</b>, <b>148</b>. The IC <b>12</b> or non-compressive interposer is not completely covered by the compressive interposer leafs <b>146</b>, <b>150</b>. While only three leaves are shown and only two are labeled to simplify the illustration and description, respectively, other embodiments could have any number of leafs.

0060<figref idref="DRAWINGS">FIGS. 20-21</figref> illustrate an alternative embodiment of the C-shaped spring leaf spacers illustrated in <figref idref="DRAWINGS">FIGS. 18-19</figref>. In this embodiment, the interposers are formed from a series of interleaved springs with C-shaped compressive sections <b>164</b>, <b>168</b>. In this embodiment, IC <b>12</b> or non-compressive interposer <b>116</b> is completely covered by the compressive interposer <b>162</b>.

0061<figref idref="DRAWINGS">FIG. 22</figref> illustrates an alternative embodiment of an interposer comprising a series of compressive sections <b>180</b>, <b>182</b>, and <b>184</b>. These interposers may be of the same type or configuration, or may be of different types or configurations. In <figref idref="DRAWINGS">FIG. 22</figref>, a C-Shaped compressive section <b>182</b> is sandwiched between two accordion-shaped compressive sections <b>180</b> and <b>184</b>. In this case, the compressive sections are welded, spot welded, brazed, soldered or otherwise thermally attached to a heat slug or to an interposer top sheet <b>122</b> as shown in <figref idref="DRAWINGS">FIG. 22</figref>.

0062<figref idref="DRAWINGS">FIG. 23</figref> is an illustration of a multi-chip IC package <b>200</b>. In this package, the substrate can support multiple ICs. Two ICs <b>202</b> and <b>204</b> are depicted in <figref idref="DRAWINGS">FIG. 23</figref>, each with its own compressive interposer(s) <b>216</b> and <b>218</b>. In the embodiment shown, the interposer <b>218</b> for one IC <b>204</b> is formed with a punch press to form the C-shape spring leaves. The interposer <b>216</b> for the other IC <b>202</b> is brazed/welded to an exposed top section <b>212</b> of the interposer <b>210</b>. The individual compressive, thermally conductive interposer can be made or assembled together with a stress release feature <b>220</b> as shown in <figref idref="DRAWINGS">FIG. 23</figref>.

0063<figref idref="DRAWINGS">FIG. 24</figref> illustrates a feature that can be included in any of the compressive interposer embodiments described above. Specifically, it illustrates a compressive section <b>224</b> between the top section <b>212</b> and of the compressive interposer <b>210</b> and a mount section <b>226</b> which rests on the substrate <b>20</b>. These features may be used for extra support or to help register the location of the compressive interposer within the package during manufacture. These features may also be combined with any of the other embodiments described herein, including embodiments <b>52</b>, <b>71</b>, <b>100</b>, <b>130</b>, <b>250</b>, <b>140</b>, <b>160</b>, <b>122</b>, or any other embodiments not specifically described herein.

0064Although a focus is made on shaped compressive interposers, other forms of shaped and unshaped compressive interposers are also contemplated as being within the scope and spirit of the disclosure.

0065The above description of the present invention is illustrative, and is not intended to be limiting. It will thus be appreciated that various additions, substitutions and modifications may be made to the above described embodiments without departing from the scope of the present invention. Accordingly, the scope of the present invention should be construed in reference to the appended claims.